Interpretive Summary: Why do some species of flowering plants host so many native bee species, and why don't these species eliminate each other over time as a result of persistent competition for identical food resources? Creosote bush, a dominant shrub of America's warm deserts, hosts over 100 species of native bees, nearly all non-social, including 22 species that rely solely on creosote bush for all of their pollen needs. These specialists have arisen in the past 20,000 years, when fossil evidence indicates creosote bush first arrived in the Northern Hemisphere. Rather than a single radiation, like Darwin's finches, these bees represent at least 16 independent evolutionary lineages. Their predominance in the harshest deserts where creosote bush grows suggests that the unpredictability of bloom, owing to frequent drought, is better accommodated by specialists that generalists. The persistence of multiple bee species on identical floral resources holds promise for the feasibility of fostering multiple pollinator species on select agricultural crops without competitive detriment.

Technical Abstract:
The 20,000 year documented history of the creosote bush in warm deserts of North America offers an unprecedented opportunity to study the evolutionary origins of foraging specializations among 120 species of native bee that visit flowers of this dominant shrub. Multiple, phylogenetically independent specialization has evolved for 22 bee taxa that use this floral host exclusively for all of their pollen needs. These specialists predominate in the harshest deserts that are home to this shrub, presumably due their abilities to detect and diapause through years of inadequate rainfall for bloom. Their specialization has not arisen in response to nutritional factors or coevolution with chemical defenses, as is with the case of other insect herbivores. The evolutionary rapidity of host switching and subsequent specialization by native bees was not expected, given the published literature for host specialization by other herbivorous insects.